The present invention relates to methods and apparatus for making measurements of the amplitude and phase of ultrashort pulses and particularly ultrashort optical pulses which have a frequency range from the far infrared through the ultraviolet and have a duration in the subpicosecond range. More particularly, the invention provides methods and instrumentalities for characterizing the electric fields of ultrashort optical pulses by spectral phase interrometry for direct electric field construction of such pulses (SPIDER).
The invention is especially suitable for use in measuring ultrashort optical pulses having a duration of less than 100 picoseconds. Such pulses are useful in 2-photon and confocal microscopy, especially at low light levels, material science in the analysis of the electronic or vibrational dynamic properties of materials, lidar and lidar signal processing, medical diagnostics, including coherence tomography, phototherapies and for the detection of signals in high speed optical communication systems. By characterizing (measuring the amplitude and phase) of ultrashort optical pulses in accordance with the invention, such pulses become especially adapted for use in the detection of quantum-limited light fields with ultra precise temporal resolution thereby reducing uncertainty and lack of resolution (dynamic imaging problems) that are photon-limited (low-light) in applications such as ranging, electronic digital photography, and medical imaging.
The present invention advances autocorrelation techniques which have heretofore been used for measuring short laser pulses. In such techniques, interferometer developed identical replicas were relatively delayed in time. These pulse replicas were then recombined and passed through a nonlinear element. The resultant signal was recorded as a function of variable time delay between the replicas. However the signal provides only an estimate of the pulse amplitude and phase. In another technique, which has become to be known as frequency resolved optical gating (FROG), a spectrometer is used to analyze the intensity and phase of the signal transmitted through the nonlinear element. Such a system has required the collection of a large quantity of data and has not been found suitable for real time, high speed measurements and characterization of ultrashort optical pulses. Further information respecting the auto correlation and FROG system may be had by reference to U.S. Pat. No. 5,530,544 to Trebino et al issued Jun. 25, 1996. Another technique for characterizing optical pulses uses a dispersed replica of the pulse, but without any frequency shift of the replica. See Fortenberry et al, U.S. Pat. No. 5,684,584 issued Nov. 4, 1997.
The present invention operates on the principle of spectral shearing interferometry in which two frequency shifted (sheared) replicas of the input light pulse to be measured are interfered for spectral analysis. The frequency shear is generated by the use of a chirp pulse by mixing the chirp pulse with replicas having a delay which is much shorter than the duration of the chirp pulse so that different frequency components of the chirp (the frequency across which the pulse sweeps) convert the pulses to different frequencies. Real time measurements are facilitated by the use of a data inversion routine in processing the spectrogram data, and provide the direct electric field reconstruction of the optical pulse being measured. Since the measurements are made interferometrically as a function of the spectral phase of the pulse, the wave field (electric field) is reconstructed, the invention provides what may be termed as spectral phase interferometry for direct electric field reconstruction (SPIDER).
It is therefore an object of the present invention to provide methods and apparatus for measurement of an ultra short pulse, especially an ultra short optical pulse, using a chirp pulse and nonlinearity (as in a nonlinear mixer) to generate spectral shear and provide a signal exhibiting spectral shear corresponding to the input pulse to be measured.
Another object of the invention is to provide a system for the characterization of ultrashort optical pulses wherein data collection is minimized and real time operation is obtained in that the analysis of a spectrally sheared signal may be carried out by a direct and rapid inversion routine.
It is still a further object of the present invention to provide method and apparatus for ultrashort optical pulse measurements which has elements, for example an interferometer a pulse disperser, a nonlinear mixer, and a spectral detector which can be disposed in a collinear arrangement.
It is a further object of the present invention to provide improved methods and apparatus for the measurement of optical pulses ranging from the ultraviolet to the infrared in which nonlinear optics is used.
It is a still further object of the present invention to by methods and apparatus for characterizing pulses by making measurements of the intensity and phase of such pulses, both in temporal and frequency space.